Congenital malformations secondary to in utero exposure to the antiepileptic drugs phenytoin and carbamazepine are amongst the most common of all drug-induced birth defects in humans. In spite of the common nature of these congenital defects, efforts to obtain a clear understanding of their pathogenesis on a molecular level have been limited by the availability of sufficient quantities of embryonic material from which to isolate mRNA and ultimately develop cDNA libraries to screen. The research program described in this proposal takes advantage of two recent advances in molecular biological techniques that can overcome this problem. Using in situ transcription (IST) and aRNA amplification, we intend to examine in murine embryos exposed in utero to two different teratogenic treatments, the composite changes in a population of mRNAs from selected candidate genes whose function is believed to be critical for normal craniofacial development and neural tube closure. The experimental paradigm will include embryos and fetuses that differ in their genetically-determined sensitivity to teratogen- induced defects as demonstrated in over 10 years of work on the model system. These experiments should provide evidence as to whether teratogenic treatments induced abnormal development by directly altering gene expression during critical periods of craniofacial and neural development. Further, it will become evident whether or not there is but a single mechanistic pathway to the development of the observed malformations, or if there are multiple pathways that different exogenous agents can exploit. Utilizing a differential hybridization screening strategy, it will be possible to isolate those sequences that are vastly enriched or in very low abundance yet remain of critical important in the pathogenesis of congenital defects. It will also be possible to use the aRNA probes from selected genotypes (strains), anatomical regions, gestational stages and teratogenic treatments to screen existing cDNA or genomic libraries using a differential amplification strategy to detect cDNAs encoding messages whose abundance changes in response to the drug treatment. The results obtained in these murine models will provide insights into the development of analogous defects in humans, which may lead to better preventative measures.